Embodiments of the present invention generally relate to the field of spectrally resolved imaging systems and, more specifically, to extending the imaging range and/or improving the signal-to-noise ratio of spatially scanning “hyperspectral” imaging systems.
Accurate measurement of spectral signatures requires sensing an image in many narrow spectral bands, also referred to herein as channels. Since each spectral channel is so narrow, each channel accumulates relatively small signals. The lower the signal levels, the lower the achievable imaging range between object and sensor. Longer imaging range is improved by collecting more signal.
Most hyperspectral sensors are scanning systems that image an object in one dimension through a narrow slit onto a sensing array while spreading out the spectrum of each point along the slit in a direction orthogonal to the slit using a prism or grating. These slit spectrometers are typically operated in a “pushbroom” scan mode. They operate by continuously scanning a single line of pixels across the scene. The light is passed through the slit and dispersed into its constituent colors by either a diffraction grating or prism within the optical spectrometer. Exposure time (i.e., signal collection time) for each pixel is extremely limited.
One method to increase the signal level in a slit based spectrometer is to increase the size of the collecting optics. Another approach is to give up (i.e., reduce) spatial resolution by allowing the individual pixels to collect more signal from a larger area. Yet another approach is to scan at much slower speeds or to backscan. Lastly, dual-slit spectrometers have been proposed.
For non-spectrometer, line scan imaging systems, benefits of Time-Delay and Integration (TDI) are well known. TDI in line-scanning systems substantially increases the signal-to-noise ratio (SNR) and results in greatly increased imaging ranges, scanning speeds, and detection sensitivities, or various combinations thereof. In the visible region of the electromagnetic spectrum, analog, two-dimensional charge-coupled devices (CCDs) based upon silicon detector technology are very commonly utilized for TDI line-scanners. Signal charges from along-scan direction pixels are transferred and accumulated from one detector to the next in synchronicity with the image moving across the array. The more stages of TDI, the more signal that is accumulated as the image moves across the array.
In the infrared spectral regime, silicon CCD detectors are not applicable, so digital TDI (dTDI) has been developed. In dTDI, fast framing arrays are utilized. Signals from the individual pixels are digitized at the end of each frame integration period. The digital values from the along-scan-direction pixels, from the current frame, are added to the accumulated value from the previous pixel from the previous frame, tracking the image motion across the array.